At first I considered plastic bags as well. But I found hardcover cases for those boosters to be more fitting.
To give a small update on my project:
Luckily I found the boxes that WestonBoxes provides on amazon.de as well. Only in packages of 8 boxes, so that I had to order 3 packages for 12€ each but that should be fine. If the data about the sizes of the boxes is correct they should be perfectly fitting for normal sleeved cards.
Right now I'm thinking of which sleeves I want to use. I think I want some cheap ones, since I'll need 400.
I'm also struggling with the idea to have printed cardlists for the boosters hidden somewhere inside the boxes, for an easy sorting apart after the matches.
As soon as all parts have arrived I'll post some pictures or a youtube-video-link of the results for those who wants to see it.